Project Summary The receptor tyrosine kinase (RTK) ErbB4 plays important roles in nervous system development and function, but the mechanisms by which this takes place remain poorly understood. All RTKs exert biological functions through their ?canonical? signaling cascades. Several years ago, we discovered that alternative splicing generates an ErbB4 isoform we named ErbB4-JMa that after sequential cleavage by tumor necrosis factor-?- converting enzyme (TACE) and then by the presenilin 1/?-secretase complex, signals directly to the nucleus through its soluble intracellular domain (sE4ICD). Furthermore, we and others found evidence suggesting that direct ErbB4 nuclear signaling mediates several key steps in brain development. However, these observations were made using cells in culture and mice with complete ErbB4 loss of function. Therefore, the roles of direct ErbB4 nuclear signaling in the intact nervous system remain undefined. To fill this gap in knowledge, we generated new mutant mice to eliminate the generation of sE4ICD without loss of ErbB4 canonical signaling. Specifically, using CRISPR-Cas9 we generated mice in which the ErbB4-JMa isoform has been rendered uncleavable by creating point mutations in the TACE cleavage site, or ErbB4-JMa isoform translation has been specifically eliminated by the introduction of point mutation that creates a premature termination codon. We propose to validate and characterize the effect of these mutations on ErbB4 cleavage and expression (Aim 1) and to determine the consequences of specific loss of ErbB4 nuclear signaling on brain development phenotypes known to be affected by loss of ErbB4 (Aim 2). These studies will not only provide insights into the biological roles of ErbB4, a molecule linked to neurodevelopmental disorders, but will also provide the first animal models that formally interrogate the roles of direct nuclear signaling by a receptor tyrosine kinase in the intact organism and will characterize the impact of a new presenilin-1/?-secretase-dependent signaling pathway on nervous system development in vivo. Given the involvement of presenilin-1/?-secretase in neurodegeneration, the link between ErbB4 nuclear signaling and genomic stability in the nervous system, the results from these studies can bring new insights into the mechanisms of Alzheimer's disease.